Abstract: A system for pumping an abrasive or corrosive fluid at high pressure and with a variable delivery rate. The system comprises raising the pressure in a non-corrosive and non-abrasive liquid in a pressure raising means comprising pump means and at least two intermediate cylinders containing floating pistons. The pressure is transferred to the abrasive or corrosive fluid in pressure transfer vessels. A valve arrangement controls the flow of non-abrasive and non-corrosive liquid to the intermediate cylinders and return means are provided for returning the floating pistons after each stroke. The valve arrangement is controlled in dependence on the positions of the floating pistons in their intermediate cylinders.The system is particularly useful for supplying clay slurry to a high pressure plate filter press.